An investigation is being conducted of those enzymes of detoxication that act on amines. Specifically, two isoenzymes of amine N-methyltransferase from rabbit liver have been obtained which catalyze, with overlapping specificity, the transfer of methyl groups from S-adenosyl-L-methionine to a large number and variety of amines. Serving as methyl acceptors are primary, secondary and tertiary amines of very different carbon skeleton that include aliphatic, aromatic and heterocyclic compounds. The presence of the enzyme in most animal organs, including brain, suggests the possibility of using pyridine derivatives as prodrugs that could pass the blood-brain barrier. Upon methylation within brain, the resultant pyridinium ion would remain within the cell to exert its pharmacological effects. It should be pointed out that toxic products are also possible: methylation of 4-phenylpyridine results in symptoms of Parkinsonism and methylation of 4,4'-aminodipyridyl yields the toxic paraquat. Both isoenzymes have been prepared in homogeneous form and have been characterized.